Hydrophilic microporous film

ABSTRACT

A hydrophobic microporous film comprising a hydrophilic microporous film and a surfactant coating of a silicon glycol copolymer which renders the coated microporous film hydrophilic. The hydrophilic microporous film of the instant invention more rapidly &#39;&#39;&#39;&#39;wets&#39;&#39;&#39;&#39; than microporous films of the prior art. A second surfactant, preferably an imidazoline tertiary amine, may also be included in the surfactant coating. The hydrophilic microporous film has particular application as a battery separator which must be rapidly &#39;&#39;&#39;&#39;wetted&#39;&#39;&#39;&#39; to generate its electrical output.

United States Patent Taskier Dec. 30, 1975 HYDROPHILIC MICROPOROUS FILM3,297,638 1/1967 Beaulieu 136/146 [751 Inventor: Henry skier, Englewood,325332 1511333 kiffiffii'j i .113331: 1321122 [73] Assignee: CelaneseCorporation, New York,

N Y Primary ExaminerD0nald L. Walton Attorne A cut, or FirmThomas J. Moran; Linn l. [22] F'led: 1974 Grim; hllarvfin Bressler g [21] Appl. No.:447,184

AB T Related [1.5. Application Data [57] STRAC Division of Ser No 245260 A I8 1972 P t A hydrophobic microporous film comprising a hydro- No3 853 60 philic microporous film and a surfactant coating of a silicon 1col co ol er which renders the coated mit'l ll d h'l' Th hd h'l' Icroporous 1m y rop me. e y rop ||c micropo- (g1; mus mm of the instantinvention more rapidly [58] h 36/146 154 than microporous films of theprior art. A second surfactant, preferably an imidazoline tertiaryamine, may [56] Reerences Cited also be included in the surfactantcoating.

UNITED STATES PATENTS The hydrophilic microporous film has particular 2700 694 1955 Femald 136/146 apphcatlon as a battery separator vhich mustbe 2,946,707 7/1960 Sperber 136/1 rapidly Wetted to generate itselectrical output. 3,239,381 3/1966 O'Connell 136/146 2 Claims, NoDrawings HYDROPHILIC MICROPOROUS FILM This is a division, of applicationSer. No. 245,260, filed Apr. 18, 1972, now U.S. Pat. No. 3,853,601.

BACKGROUND OF THE DISCLOSURE 1. Field of the Invention The instantinvention is directed to a hydrophilic microporous film and the processfor making same. More particularly, the instant invention is directed toa hydrophilic microporous film comprising a hydrophobic microporous filmand a surfactant coating and the process for making the film. Still moreparticulary, the instant invention is directed to a battery separatorcom prising a hydrophobic microporous film rendered hydrophilic by asurfactant coating of a silicon glycol copolymer and the process formaking said separator.

2. Background of the Prior Art Recent developments in but not limitedto, the area of microporous polymeric film, exemplified by U.S. Pat. No.3,426,754, issued on Feb. 11, 1969, U.S. Pat. No. 3,558,764, issued onJan. 26, 1971, copending U.S. application, Ser. No. 867,425, filed onNov. 13, 1969, U.S. Pat. No. 3,679,538, issued on July 25, 1972 andcopending U.S. patent application, Ser. No. 104,715 filed on Jan. 7,1971 and now abandoned have instigated studies to discover applicationswhich could ex' ploit the unique properties of these new films.

One disadvantage of these films, which in the past has limited thenumber of applications to which these films many be put, has been thehydrophobic nature of most of these films. This is especially true whenpolyolefinic films, a preferred type of polymeric material oftenemployed in the manufacture of microporous films are employed. Becausethey would not be wetted" with water and aqueous solutions they couldnot be used in such logical applications as filter media and the like.Fortunately, this major drawback to the use of microporous films hasbeen overcome with the discovery that these films may be made to wetwith water and aqueous media, or alternatively stated, be madehydrophilic by coating with surfactant. The specifics of this discoveryis disclosed in copending U.S. application, Ser. No. 106,564, filed onJan. 14,1971.

That hydrophilic microporous films may be produced has suggested anapplication where microporous films would present a dramatic improvementover the current state of the art. That is, as a battery separator. Abattery separator is a critical component of a battery. A battery iscomprised of one or more electrolytic cells enclosed by a housing. Eachcell includes two electrical terminal or electrodes, the anode and thecathode. The electrodes are immersed in a conducting medium, theelectrolyte. Electrical current flows between the electrodes. Thiselectrical current results from the flow of electrons across an externalcircuit electrolyte. Just as electrons, flow across the external circuitso do ions charged species, flow in the electrolyte. Although it isabsolutely essential to the production of an electrical current thations flow between electrodes in the electrolyte it is usuallydetrimental in a battery for anions, positively charged ions, to flow tothe anode, the negatively charged terminal, since this decreases thedriving force. to prevent deleterious positive ionic flow of one or morespecies between terminals, a function of a battery separator. Thebattery separator is disposed in the electrolytic cell between the anodeand cathode of the electrolyte to prevent deleterious ion migration.

The above description suggests the type of material that should ideallybe used as a battery separator. An excellent battery separator is onewhich has pore open ings which are small enough to prevent largespecies, such as large positive ions to flow through its pores yet largeenough to permit the flow of cations, negatively charged ions throughthese pores. Similary, the battery separator should be of minimumthickness in view of the well known fact that the flow of ions across abattery separator is inversely proportional to the thickness of theseparator.

Another requirement of a battery separator suggests itself when oneconsiders that the electrolytes employed in most battery applicationsare highly basic or acidic. A good battery separator should be inert, tothese highly corrosive materials.

A final requirement for a good battery separator is that the separatorbe wetted by the electrolyte employed. In view of the fact thatessentially all of the electrolytes currently utilized are aqueoussolutions, this requirement necessitates that the battery separator behydrophilic. The battery separator must be totally wetted so as toprovide a continuous electrolytic path on either side of the batteryseparator to permit the flow of ions therethrough. An analogy can bedrawn to an electrically conducting wire. A break in the wire cuts offthe flow of electrons. So, in the case of an electrolyte, thenon-wetting of a portion of a battery separator effectively cuts off thepath for ionic flow over the non wetted area, thus, cutting down on theoutput of the battery.

In the prior art many battery separators have been advanced. Many of theseparators excellently furnish one or more of the necessarycharacteristics. However, few if any are excellent in each of thesecharacteristics. For example, cellulosic films possess a pore size whichpermits the flow of cations while preventing deleterious anionic flow.Cellulosics are reasonably satisfactory in terms of minimum thickness.However, their resistance to strong acids and bases are minimal and theyoften fail relatively due to chemical attack. Specially treatedpolyvinyl chloride films have good resistance to chemical attack.However, their thickness makes for high electrical resistance thusdecreasing the electrical output of the battery.

As stated above copending U.S. application, Ser. No. 106,564 teachessurfactant coating of a hydrophobic microporous film. Thus any of thesurfactants suggested in the copending application will provide ahydrophilic film. Seemingly, this should be enough to ipso facto producea satisfactory battery separator. Unfortunately, in addition toabove-discussed criteria for battery separators, an additional criterionin the form of a further restriction on wettability is imposed. That is,wettability must result immediately. Thus water and aqueous solutionsmust pass through the pores of the battery separator immediately aftercontact therewith. Complete wettabiliy, as stated above, is essentialfor the generation of the rated electrical output of the battery. Asthose skilled in the art are aware battery manufacturers immediatelytest their batteries after fabrication. This immediate test is made topermit the manufacturer to determine the proper functioning of hisbatteries. A manufacturer cannot wait long periods for the batteryseparator to wet and thus conduct an electrical current. This wouldrequire immense storage area or diminished production levels. A moredramatic example of the need for immediate wettability is provided bybatteries of the type which are not filled with electrolyte until thebattery is purchased. Obviously, a purchaser is not in a position towait a few days for wettability to be established to permit thenecessary electrical output.

This further requirement for battery separators, rapid wettability isnot provided in the surfactant coated microporous polymer film disclosedin copending application, Ser. No. 106,564. It has been found that noneof the surfactant coatings suggested therein provide the combination ofproperties, including rapid wettability necessary in battery separators.This result is to be expected in that application, Ser. No. 106,564 isdirected primarily to applications where a pressure gradient is imposedacross the coated film, i.e., as a filter. The imposition of a pressuregradient compensates for low wettability, that is, the pressureovercomes the surface tension associated with hydrophobic surfaces topermit the flow of aqueous liquids across the porous surface of thecoated film. Unfortunately, a battery separator is an example of anapplication where no positive pressure gradient is or can be imposedacross the separator.

That the copending application does not anticipate battery separators asan application for the coated films therein disclosed is evidenced bythe failure to enumerate battery separators as an application of thatcoated microporous film invention.

BRIEF SUMMARY OF THE INVENTION The instant invention is directed to ahydrophilic microporous film comprising a surfactant coated hydrophobicmicroporous polymeric film and a process for forming it. The hydrophilicfilm of the instant inven tion has properties which include: a pore sizesufficiently small to bar the flow of deleterious anions but largeenough to permit the flow therethrough of cations; a small enoughthickness to minimize the resistance caused by ionic flow thereacross; achemical inertness sufficient to resist chemical attack by strong acidsand bases of the type used as electrolytes; and a high level ofwettability which is provided almost immediately after contact withwater and water base liquids. These properties make the hydrophilic filmof the instant invention an excellent battery separator.

In accordance with the instant invention a hydrophilic microporous filmis provided comprising a hydrophobic microporous film, characterized byhaving a reduced bulk density as compared to the bulk density of thecorresponding non-porous precursor film from which it is formed, acrystallinity of above about 30 percent, an average pore size of about100 to 12,000 Angstroms, a surface area of about 2 to about 200 metersper gram, and a void volume of 20 to 45 percent. The battery separatorof the instant invention also includes a coating of a silicon glycolcopolymer surfactant. Preferably the surfactant represents 2 to 20percent by weight of the uncoated microporous film. In a preferredembodiment of the instant invention, the microporous film of the instantinvention is coated with a coating which comprises a silicon glycolcopolymer with a second surfactant, preferably an imidazoline tertiaryamine.

The instant invention is also directed to a process for forming ahydrophiliic microporous film of the instant invention. In the processof the instant invention, a microporous polymeric film having theproperties enumerated above is coated with a surfactant coating com- 4prising a silicon glycol copolymer to form the battt y separatordescribed above. In a preferred embodiment the surfactant coatingincludes, along with the silicon glycol copolymer, a second surfactant,an irnidazoline tertiary amine.

DETAILED DESCRIPTION Porous or cellular films can be classified into twogeneral types: one type in which the pores are not interconnected, i.e.,a closed-cell film, and the other type in which the pores areessentially interconnected through tortuous paths which may extend fromone exterior surface or surface region to another, i.e., an opencelledfilm. The porous films of the present invention are of the latter type.

Further, the pores of the porous films of the present invention aremicroscopic, i.e., the details of their pore configuration orarrangement are discernible only by microscopic examination. In fact,the open cells or pores in the films generally are smaller than thosewhich can be measured using an ordinaary light microscope, because thewavelength of visible light, which is about 5,000 Angstroms (an Angstromis one tenbillionth of a meter), is longer than the longest planar orsurface dimension of the open cell or pore. The microporous films of thepresent invention may be identified, however, by using electronmicroscopy techniques which are capable of resolving details of porestructure below 5,000 Angstroms.

The microporous films of the present invention invention are alsocharacterized by a reduced bulk density, sometimes hereinafter referredto simply as a low" density. That is, these microporous films have abulk or overall density lower than the bulk density of correspondingfilms composed of identical polymeric material but having noopen celledor other voidy structure. The term bulk density" as used herein meansthe weight per unit of gross or geometric volume of the film, wheregross volume is determined by immersing a known weight of the film in avessel partly filled with mercury at 25C. and atmospheric pressure. Thevolumetric rise in the level of mercury is a direct measure of the grossvolume. This method is known as the mercury volumenometer method, and isdescribed in the Encyclopedia of Chemical Technology, Vol. 4, page 892(lnterscience I949).

Porous films have been produced which possess a microporous, open-celledstructure, and which are also characterized by a reduced bulk density.Films possessing this microporous structure are described, for example,in US. Pat. No. 3,426,754 which patent is assigned to the assignee ofthe present invention. This preferred method of preparation describedtherein involves drawing or stretching at ambient temperatures, i.e.,cold drawing," a crystalline, elastic starting film in an amount ofabout ID to 300 percent of its original length, with subsequentstabilization by heat setting of the drawn film under a tension suchthat the film is not free to shrink or can shrink only to a limitedextent.

While the above described microporous or void-containing film of theprior art is useful in this invention the search has continued for newprocesses able to produce open-celled micoporous films having a greaternumber of pores, a more uniform pore concentration of distribution, alarger total pore area, and better thermal stability of the porous orvoidy film. These properties are significant in applications such aswhen used as a battery separator where a large number of uniformlydistributed pores are necessary or highly desirable.

It is submitted however that the process disclosed in U.S. Pat. No.3,426,754 and the improvement thereof, hereinafter defined in detail,are equally capable of producing a microporous product which may berendered hydrophilic by this invention.

An improved process for preparing open-celled microporous polymer filmsfrom non-porous, crystalline, elastic polymer starting films, includes(1 cold stretching, i.e., cold drawing the elastic film until poroussurface regions or areas which are elongated normal or perpendicular tothe stretch direction are formed, (2) hot stretching, i.e., hot drawing,the cold stretched film unitl fibrils and pores or open cells which areelongated parallel to the stretch direction are formed, and thereafter(3) heating or heat setting the resulting porous film under tension,i.e., at substantially constant length, to impart stability to the film.Yet another process is similar to this process but consolidates steps(2) and (3) into a continuous, simultaneous, hot stretchingheat settingstep, said step being carried out for a time sufficient to render theresulting microporous film substantially (less than about percent)shrink resistant.

The elastic starting film or precursor film is preferably made fromcrystalline polymers such as polypropylene or other polyolefins by meltextruding the polymer into a film, taking up the extrudate at a drawdownratio giving an oriented film, and thereafter heating or annealing theoriented film if necessary to improve or enhance the initialcrystallinity.

The essence of the improved processes is the discovery that thesequential cold stretching and hot stretching steps impart to theelastic film a unique open-celled structure which results inadvantageous properties, including improved porosity, improved thermalstability and a gain or enhancement of porosity when treated withcertain organic liquids such as perchloroethylene.

As determined by various morphological techniques or tests such aselectron microscopy, the microporous films of the improved process arecharacterized by a plurality of elongated, non-porous, interconnectingsurface regions or areas which have their access of elongationsubstantially parallel. Substantially alternating with a defined bythese non-porous surface regions are a plurality of elongated, poroussurface regions which contain a plurality of paralled fibrils or fibrousthreads. These fibrils are connected at each of their ends to thenon-porous regions, and are substantially perpendicular to them. Betweenthe fibrils are the pores or open cells of the films utilized by thepresent invention. These surface pores or open cells are substantiallyinterconnected through tortuous paths or passageways which extend fromone surface region to another surface area or region.

With such a defined or organized morphological structure, the films ofthe present invention may have a greater proportion of surface area thatthe pores cover, a greater number of pores, and a more uniformdistribution of pores, than previous microporous films. Further, thefibrils present in the films of the present inven- Elastic Recovery (ER)tion are more drawn or oriented with respect to the rest of the polymermaterial in the film, and thus contribute to the higher thermalstability of the film.

In fact the total surface area per gram of material of the films of thisinvention, as determined by the BET Method which is described in detailin the Journal of the American Chemical Society, Vol. 60, pp. 309-316(1938), is in the range of from 2 to about 200 square meters per gram.Preferably the range is from about 5 to about 100 square meters per gramand most preferably from about 20 to about 60 square meters per gram.These values can be compared with normal pin-holed film which has atotal surface area per gram of about 0.1 square meter; paper and fabricwhich have values per gram of about 1.0 square meters and leather whichhas a value of about l.6 square meters per gram. Additionally, thevolume of space per volume of material range from about 0.05 to about1.5 cubic centimeters per gram and most preferably from 0.2 to about0.85 cubic cc. per gram. A more convenient way of expressing thisporosity property is as a fraction of the total film volume. Thus, thevoid volume is in the range of between 20 and 45 percent, as determinedby gravimetric methods. More preferably, the percent void volume is inthe range of 25 to 35 percent. Additional data to define the films ofthis invention relates to nitrogen flux measurements, wherein themicroporous films have Q (or nitrogen) Flux values in the range of fromabout 5 to 400 preferably about 50 to 300.

Nitrogen flux may be calculated by mounting a film having a standardsurface area of 6.5 square centimeters in a standard membrane cellhaving a standard volume of 63 cubic centimeters. The cell ispressurized to a standard differential pressure (the pressure dropacross the film) of 200 pounds per square inch with nitrogen. The supplyof nitrogen is then closed off and the time required for the pressure todrop to a final differential pressure of 150 pounds per square inch asthe nitrogen permeates through the film is measured with a stop watch.The nitrogen flux, Q, in gram moles per square centimeter minute, isthen determined from the equation:

27.74 X 10 Q: Ar T where A t is the change in time measured in secondsand T is the temperature of nitrogen in degrees Kelvin. The aboveequation is derived from the gas law,

The microporous films of the present invention are formed from astarting elastic film of crystalline, filmforming, polymers. Theseelastic films have an elastic recovery at zero recovery time(hereinafter defined) when subjected to a standard strain (extension) of50 percent at 25C. and 65 percent relative humdity of at least about 40percent, preferably at least about 50 percent, and most preferably atleast about percent.

Elastic recovery as used herein is a measure of the ability of astructure or shaped article such as a film to return to its originalsize after being stretched, and may be calculated as follows:

(length when stretched) (length after stretching X lOO) length addedwhen stretched Although a standard strain of 50 percent is used toidentify the elastic properties of the starting films, such strain ismerely exemplary. In general, such starting 7 films will have elasticrecoveries higher at strains less than 50 percent, and somewhat lower atstrains substantially higher than 50 percent, as compared to theirelastic recovery at a 50 percent strain.

These starting elastic films will also have a percent crystallinity ofat least 20 percent, preferably at least 30 percent and most preferablyat least 30 percent and most preferably at least 50 percent, e.g., about50 to 90 percent, ore more. Percent cyrstallinity is determined by theX-ray method described by R. G. Quynn et al. in the Journal of AppliedPolymer Science, Vol. 2, No. pp. l66-l73 (l959). For a detaileddiscussion of crystallinity and its significance in polymers, seePolymers and Resins, Golding (D. Van Nostrand, 1959).

Preferred suitable starting elastic films, as well as the preparationthereof, are further defined in British Pat. No. 1,198,695, publishedJuly 15, 1970. Other elastic films which may be suitable for thepractice of the present invention are described in British Pat. No.1,052,550, published Dec. 2l, 1966 and are well known in the art.

The starting elastic films utilized in the preparation of themicroporous films of the present invention should be differentiated fromfilms formed from classical elastomers such as the natural and syntheticrubbers. With such classical elastomers the stress-strain behavior, andparticluarly the stress-temperature relationship, is governed byentropy-mechanism of deformation (rubber elasticity). The positivetemperature coefficient of the retractive force, i.e., decreasing stresswith decreasing temperature and complete loss of elastic properties asthe glass transition temperatures, are particularly consequences ofentropy-elasticity. The elasticity of the starting elastic filmsutilized herein, on the other hand, is of a different nature. Inqualitative thermodynamic experiments with these elastic starting films,increasing stress with decreasing temperature (negative temperaturecoefficient) may be interpreted to mean that the elasticity of thesematerials is not governed by entropy effects but dependent upon anenergy term. More significantly, the starting elastic films have beenfound to retain their stretch properties at temperatures where normalentropy-elasticity could no longer be operative. Thus, the stretchmechanism of the starting elastic films is thought to be based onenergy-elasticity relationships, and these elastic films may then bereferred to as non-classical" elastomers.

As stated, the starting elastic films employed in this invention aremade from a polymer of a type capable of developing in significantdegree of crystallinity, as contrasted with more conventional or"classical" elastic materials such as the natural and synthetic rubberswhich are substantially amorphous in their unstretched or tensionlessstate.

As significant group of polymers, i.e., synthetic resinous materials, towhich this invention may be applied are the olefin polymers, e.g.,polyethylene, polypropylene, poly-3-methyl butene-l poly-4-methylpentene-l, as well as copolymers of propylene, 3-methyl butene-l4-methyl pentene-l, or ethylene with each other or with minor amounts orother olefins, e.g., copolymers of propylene and ethylene, copolymers ofa major amount of 3-methyl butene-l and a minor amount of a straightchain n-alkene such as n-octene-l n-hexadecen-l n-octadecene-l or otherrelatively long chain alkenes, as well as copolymers of 3-methylpentene-l and any of the same n-alkenes mentioned previously inconnection with 3-methyl butene-l. These polymers in the form of filmshould generally have a present cyrstallinity of at least 20 percent,preferably at least 30 percent, and most preferably about 50 percent topercent or higher. In the preferred films of the instant invention, thecrystallinity ranges between about 30 to about 60 percent and morepreferably between 50 and about 60 percent.

For example, a film-forming homopolymer of polypropylene may beemployed. When propylene homopolymers are contemplated, it is preferredto employ an isotactic polypropylene having a percent crystallinity asindicated above, a weight average molecular weight ranging from about100,000 to 750,000 preferably about 200,000 to 500,000 and a melt index(ASTM- l958D-l238-57T, Part 9, page 38) from about 0.l to about 75,preferably about 0.5 to 30, so as to give a final film product havingthe requisite physical properties.

While the present disclosure and examples are directed primarily to theaforesaid olefin polymers the invention also contemplates the highmolecular weight acetal, e.g., oxymethylene, polymers. While both acetalhomopolymers and copolymers are contemplated, the preferred acetalpolymer is at random" oxymethylene copolymer, on which containsrecurring oxymethylene, i.e., CH O, units interspersed with -OR-- groupsin the main polymer chain where R is a divalent radical containing atleast two carbon atoms directly linked to each other and positioned inthe chain be tween the two valences, with any substituents on said Rradical being inert, that is, those which do not include interferingfunctional groups and which will not induce undesirable reactions, andwhere a major amount of the OR units exist as single units attached tooxymethylene groups on each side. Examples of preferred polymers includecopolymers of trioxane and cyclic ethers containing at least twoadjacent carbon atoms such as the copolymers disclosed in U.S. Pat. No.3,027,352 of Walling et a1. These polymers in film form may also have acrystallinity of at least 20 percent, preferably at least 30 percent,and most preferably at least 50 percent, e.g., 50 to 60 percent orhigher. Further, these polymers have a melting point of at least C, anda number average molecular weight of at least 10,000. For a moredetailed discussion of acetal and oxymethylene polymers, seeFormaldehyde, Walker, pp. l75-l9l, (Reinhold 1964).

Other relatively crystalline polymers to which the invention may beapplied are the polyalkylene sulfides such as polymethylene sulfide andpolyethylene sulfide, the polyarylene oxides such as polyphenyleneoxide, the polyamides such as polyhexamethylene adipamide (nylon 66) andpolycaprolactam (nylon 6), and polyesters such as polyethyleneterephthalate, all of which are well known in the art and need not bedescribed further herein for sake of brevity.

The types of apparatus suitable for forming the starting elastic filmsof this invention are well known in the art.

For example, a conventional film extruder equipped with a shallowchannel metering screw and coat hanger die, is satsifactory. Generally,the resin is introduced into a hopper of the extruder which contains ascrew and a jacket fitted with heated elements. The resin is melted andtransferred by the screw to the die from which it is extruded through aslot in the form of a film from which it is drawn by a take-up orcasting roll. More. than one take-up roll in various combinations orstages may be used. The die opening or slot width may be in the range,for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawdown ratioof about 20:1 to 200:1, preferably 50:1 to 150:1.

The terms drawdown ratio" or, more simply, draw ratio, as used herein isthe ratio of the film wind-up or take-up speed to the speed of the filmissuing at the extrusion die.

The melt temperature for film extrusion is, in general, not higher thanabout 100C. above the melting point of the polymer and not lower thanabout 10C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature ofabout 180 to 270C., preferably 200 to 240C. Polyethylene may be extrudedat a melt temperature of of about 175 to 225C., while acetal polymers,e.g., those of the type disclosed in U.S. Pat No. 3,027,352 may beextruded at a melt temperature of about l85 to 235C, preferably 195' to215C.

The extrusion operation is preferably carried out with rapid cooling andrapid drawdown in order to obtain maximum elasticity. This may beaccomplished by having the take-up roll relatively close to theextrusion slot, e.g., within 2 inches and, preferably within 1 inch. Anair knife" operating at temperatures between, for example and 40C., maybe employed within 1 inch of the slot to quench, i.e., quickly cool andsolidify the film. The take-up roll may be rotated, for example, at aspeed of to 100 ft/min, preferably 50 to 500 ftlmin.

While the above description has been directed to slit die extrusionmethods, an alternative method of forming the starting elastic filmscontemplated by this invention is the blown film extrusion methodwherein a hopper and an extruder are employed which are substantiallythe same as in the slot extruder described above. From the extruder, themelt enters'a die from which it is extruded through a circular slot toform a tubular film having an initial diameter D Air enters the systemthrough an inlet into the interior of said tubular film and has theeffect of blowing up the diameter of the tubular film to a diameter D,.Means such as air rings may also be provided for directing the air aboutthe exterior of extruded tubular film so as to provide quick andeffective cooling. Means such as cooling mandrel may be used to cool theinterior of the tubular film. After a short distance during which thefilm is allowed to completely cool and harden, it is wound up on atake-up roll.

Using the blown film method, the drawdown ratio is preferably :1 to200:1, the slot opening 10 to 200 mils, the D,/D ratio, for example, 0.5to 6.0 and preferably about 1.0 toabout 2.5, and the take-up speed, forexample 30 to 700 ftlmin. The melt temperature may be within the rangesgiven previously for straight slot extrusion.

The extruded film may then be initially heat treated or annealed inorder to improve crystal structure, e.g., by increasing the size of thecrystalline and removing imperfections therein.

The resulting partly crystalline film is then preferably subject to aprocess generally comprising either the consecutive steps of coldstretching, hot stretching and heat setting or the consecutive steps ofcold stretching and simultaneously hot stretching and heat setting. Ofcourse, less preferably variations on this process (such as theelimination of the hot stretching step) can be 10 carried out resultingin microporous films which, although inferior to those films made by thecold stretch hot stretch heat set process, still find utility as themicroprous films of this invention.

The term "cold stretching" as used herein is defined as stretching ordrawing a film to greater than its original length and at a stretchingtemperature, i.e., the temperature of the film being stretched, lessthan the temperature at which the melting of the film begins when thefilm is uniformly heated from a temperature of 25C. at a rate of 20C.per minute. The terms hot stretching" or hot stretching-heat setting" asused herein is defined as stretching above the temperature at whichmelting begins when the film is heated from a temperature of 25C. at arate of 20C. per minute, but below the normal melting point of thepolymer, i.e., below the temperature at which fusion occurs. Forexample, using polypropylene elastic film, cold stretching is carriedout preferably below about C. while hot stretching or hotstretching-heat setting is carried out above this temperature.

When a separate heat setting step is employed, it follows the coldstretching-heat stretching steps and is carried out at from about C. upto less than the fusion temperature of the film in question. Forpolypropylene the range preferably is about to about 160C.

The resulting microporous film exhibits a final crystallinity ofpreferably at least 30 percent, more preferably about 50 to 100 percentas determined by the aforementioned X-ray method. Furthermore, this filmexhibits an average pore size of about 100 to 12,000 Angstroms moreusually to 5,000 Angstroms, the values being determined by mercuryporosimetry as described in an article by R. G. Quynn et al., on pages21-34 of Textile Research Journal, January 1963.

As stated herein, it was surprisingly found in U.S. application, Ser.No. 106,564 filed on Jan. 14, 1971 that by treating a normallyhydrophobic microporous surface, such as a film of the type describedhereinbe fore, with a surfactant causes the surface to becomehydrophilic. This result, as indicated in the copending application, isparticularly surprising in view of the lack of any chemical reactionbetween the surfactant and the hydrophobic surface. (The term"hydrophobic" is defined as meaning a surface which passes less thanabout 0.010 milliliter of water per minute per sq.cc. of flat filmsurface under a water pressure of 100 psi. Likewise the term"hydrophilic" is meant to be applied to those surfaces which passgreater than about 0.01 milliliter of water per minute per sq.cc.).

As stated in the copending application, Ser. No. 106,564, surfactantswhich when applied to a film produce a film which exhibits a contactangle with water of less than about 80, preferably about 60 are suitablefor making such a film hydrophilic (The contact angle is defined as theangle between the coated surface and the tangent to a drop of waterwhich has been applied to the surface at its point of contact with thesurface, where the contact angle is measured with the film before it hasbeen rendered microporous).

Although the surfactants disclosed in copending application, Ser. No.106,5 64, all form hydrophilic microporous films when coated on thesurface of the hydrophobic film substrate, none of them were suitable asbattery separators for the reasons given above. In summary, thehydrophilic microporous film, in order to be suitable as a batteryseparator must wet, immediately 1 1 so that the battery can provide itsrated electrical output immediately.

Surfactant coated microporous films have now been unexpectedly foundwhich provide all the desirable features of the surfactant coatedmicroporous films of the prior art but which additionally possess theproperty of almost immediately wetting when placed in contact withaqueous media. Because of these properties the surfactant coatedhydrophilic microporous film of the instant invention is uniquely suitedfor use as a battery separator.

The hydrophilic microporous film of the instant invention comprises ahydrophobic micoporous poly meric film made by the procedures enumeratedin detail above. This microporous film of the instant invention has areduced bulk density as compared to the bulk density of thecorresponding non-porous precursor film. The crystallinity of the film,as stated above, is above about 30 percent. More preferably, thecrystallinity of the microporous film is in the range of about 30percent to 70 percent. Still more preferably the crystallinity is in therange of 50 percent to 70 percent. The pores of porous films of thepresent invention are microscopic, and are preferably in the range of100 to 12,000 Angstroms. More preferably, the pores are no larger than5,000 Angstroms. The microporous film of the instant invention has, aspreviously stated, a total surface area of from 2 to 200 square metersper gram. More preferably, the microporous film of the instant inventionhas a surface area in the range of from to 60 square meters per gram.The void volume of the microporous film, mentioned previously is in therange of about between 20 percent and 45 percent. More preferably, thevoid volume is in the range of about between percent and percent. in apreferred embodiment, the polymeric material from which the microporousfilm of the instant invention is formed is polypropylene.

in a preferred embodiment of the instant invention, the hydrophilicmicroporous polymeric film of the in stant invention has a preferredthickness of about 1 mil (0.001 inch). As stated above, the lesser thethickness of a hydrophilic microporous film, when employed as a batteryseparator the lower the resistance to electrical flow in the cell inwhich it is disposed. Thus, in turn, it is probably due to increaseddifficulty of maintaining electrolyte continuity through the porousseparator with increased separator thickness. In addition, a thinnerbattery separator permits less volume to be occupied by each of thecells of the battery which increases compactness of the battery. Thelesser the volume of the cell, the closer are the electrodes to oneanother. As those skilled in the art are aware, the closer the anode andcathode are to each other the more efficient the operation of theelectrolytic cell.

The hydrophilic microporous film of the instant invention also comprisesa surfactant coating which renders the hydrophobic microporous filmhydrophilic. The surfactant coating comprises a silicon glycolcopolymer. More specifically, the preferred class of silicon glycolcopolymers employed is polyoxyethylene polymethyl siloxane. Coating themicroporous polymeric film of the instant invention with the preferrednon-ionic silicon glycol polymer surfactant of the instant inventionsurprisingly renders the microporous film hydrophilic in a time muchshorter than the surfactant coatings of the prior art. This is evidencedby the dramatic decrease in resistivity of a microporous film 12 coatedwith a silicon glycol copolymer when dispos in an electrolytic cellfilled with a strong electrolyte as compared with surfactant coatedmicroporous films of the prior art.

The hydrophilic microporous film of the instant invention, may comprisein another preferred embodiment, a surfactant which comprises inaddition to a silicon glycol copolymer a second surfactant. This secondsurfactant blended with the silicon glycol copolymer is in a preferredembodiment an imidazoline. lmidazolines are tertiary amines. Thecationic imidazoline surfactants in combination with silicon glycolcopolymer surfactants produce excellent results is indeed unexpected inview of the inadequacy of imidazoline surfactants alone to produce therapid wettability necessary in battery separator applications.

in a preferred embodiment the quantity of surfactant coated onto themicroporous film represents about 2 to 20 percent by weight of theuncoated microporous film substrate. More preferably, this "add-on is inthe range of about 9 to 15 percent by weight of the uncoated microporousfilm substrate. Still more preferably, the quantity of surfactant coatedonto the surface of the microporous film, the add-on," represents about10 to 13 percent by weight of the uncoated microporous film substrate.

The instant invention also includes a process for forming a hydrophilicmicroporous film. in the process of the instant invention, a hydrophobicmicroporous film, having the properties discussed above, is coated witha surfactant coating comprising a silicon glycol surfactant, preferablypolyoxyethylene polymethyl si loxane, or a combination of a siliconglycol with a second surfactant, preferably an imidazoline to form thehydrophilic microporous film of the instant invention. Any of the wellknown coating methods may be employed to coat the ,microporous film. Onepreferred method is reverse roll coating. In this method a doctor rollis disposed partially in a bath of the surfactant coating solution. Asecond driven roll guides the uncoated hydrophobic microporous film webthrough the nip fonned by itself and the doctor roll. The two rollswhich are preferably separately driven, rotate in the same direction sothat the coated film web is guided in the direction from whence theuncoated film originates. The amount of surfactant coating disposed onthe film is a function of the difference in speed of the doctor roll andthe second film driving roll and also the size of the nip formed by thetwo rolls.

In a second preferred embodiment the squeeze roll method is used. Thefilm in this method is guided into a bath of the surfactant solution andsqueezed between two squeeze rolls disposed downstream thereof. Theamount of coating is thus a function of the gap size between the twosqueeze rolls and the pressure exerted therebetween.

A third preferred method of coating the substrate hydrophobicmicroporous film is the wire wound metering rod method. This method isthe same as the squeeze roll method except that the microporous filmafter being coated by being guided through a bath of the surfactantsolution is squeezed between a pair of wire wound metering rods whichcontrol the amount of coating disposed thereon by the configuration ofthe wires wound around the metering rods.

ln the above three methods the amount of coating is a function of one ortwo variables discussed in the description of the methods. in addition,in all three methods the amount of surfactant coating coated onto thefilm is also a function of the strength of the surfactant solution. Thesurfactant, whether it be silicon glycol copolymer alone or siliconglycol copolymer with a second surfactant is formed into a solution bythe addition of a common organic solvent such as acetone, methanol,ethanol, isopropynol or the like. The solution is preferably dilute. Ina preferred embodiment, the surfactant solution represents 1 to lpercent by weight of surfactant. More preferably, the surfactantsolution, is about 5 to percent by weight of the surfactant. Still morepreferably the surfactant solution strength is 7 percent by weight ofthe surfactant.

The following examples are given to illustrate the present invention.Because these examples are given for illustrative purposes only, theyare not intended and should not be construed as limiting the inventionin any way.

EXAMPLE 1 Polypropylene resin having a melt index of about 0.7 and adensity of about 0.92 was melt extruded at 230C. through an 8 inch slitdie of the coat hanger type using a 1 inch extruder with a shallowmetering screw. The length to diameter ratio of the extruder barrel was24:1. The extrudate was drawndown very rapidly at a melt drawdown ratioof 150, and contacted with a rotating casting roll maintained at 50C.,0.75 inches from the lip of the die. The non-porous precursor filmproduced in this fashion was found to have the following properties:thickness, 1 mil (0.00] inch); recovery from 50 percent elongation at25C., 50.3 percent; crystallinity, 59.6 percent.

A sample of this film was oven annealed in an air atmosphere with aslight tension at 140C. for about 30 minutes, removed from the oven andallowed to cool. It was found to have the following properties: recoveryfrom 50 percent elongation at 25C., 90.5 percent; crystallinity, 68.8percent.

The annealed elastic film was then cold drawn at 25C. after which thefilm was hot drawn at I45C. to produce a total draw (extension inlength) of 100 per cent. The extension ratio was 0.9, that is, 10percent of the stretching resulted from the cold drawing step and 90percent of the stretching was a result of the hot drawing step. The coldand hot drawing step resulted in the formation of micropores. Themicroporous film was thereafter heat set under tension, i.e., atconstant length, at 145C. for ID minutes in air to produce thepolypropylene microporous substrate film of the instant invention.

EXAMPLES [l TV A representative length of the microporous substrate filmmade in accordance with Example I was coated by the reverse roll coatingmethod employing a 7 percent by weight solution of an imidazolinetertiary amine surfactant, Emcol AL-42- 12, an imidazoline surfactantproduced by Witco Company, in acetone. The final 14 coated microporousfilm comprised 97 percent of the surfactant measured as a percent byweight of the uncoated substrated film.

The same procedure was repeated with the 7 percent imidazolinesurfactant solution replaced with a 7 percent by weight solution ofDow-Corning 470A fluid, a non-ionic water soluble silicon glycolcopolymer, more specifically, a polyoxyethylene polymethyl siloxane.Again, the coating procedure comprised the reverse roll coating methodand the 7 percent by weight solution of the silicon glycol copolymersurfactant again employed acetone as the solvent. The surfactant coatingrepresented 8.2 percent by weight of the uncoated microporous film ormore simply stated 8.2 percent add-on.

A third sample of the uncoated microporous polypropylene film was coatedby the same procedure as specitied for the second sample again employingDow-Corning 470A surfactant. in this case a 14.0 percent add-on of thesilicon glycol copolymer resulted.

The three coated microporous films were cut into shapes emulating thesize of typical battery separators. They were then individually testedin a electrolyte commonly employed in batteries, a 40 percent potassiumhydroxide solution. As those skilled in the art are aware, alkalinebatteries all employ, as electrolytes, a 25 to 45 percent solution ofKOH. (Additionally, these solutions sometimes include small quantitiesof lithium hydroxide.) Among the battery types that employ potassiumhydroxide solutions are high performance batteries such asnickel-cadmium, nickel-iron, silver-zinc, silver-cadmium, manganese-zincand mercury zinc.

The test consisted of imposing a known electrical potential between theanode and cathode disposed in the electrolyte, a 40 percent by weightsolution of KOH in water. The electical current generated was measuredand the resistance of the electrolytic cell was determined by Ohms Law.The coated film sample acting as a battery separator was then disposedbetween the anode and cathode and the same electrical potential wasimposed across the electrodes. The generated electrical current wasagain measured and the resistance was again calculated by using Ohm'sLaw. The second resistance calculated was, of course, higher than thefirst measured resistance. The difference between the two valuesrepresented the resistance due to the coated microporous film acting asa battery separator.

The second measurement, the measurement in which the coated film isdisposed between the electrodes in the cell, was taken I hour afterplacing the coated film battery separator into the electrolyte. The testwas repeated 24 hours after disposing the battery separator into thecell. The test was repeated for a third time 13 days after disposing theseparator into the cell. The results of this test appear in Table I. Itshould be noted that the electrical resistance is reported inmilliohmssquare inches. The area term, square inches, is used so as tomake possible comparisons for different size battery separators.

TABLE l-continued Electrical Resistance" Example Surfactant %Add-on lHour 1 Day 13 Days IV Silicon Glycol 14.0% I74 I72 I28 Percent by weightof the uncoated film. Overload, resistance beyond range of instrument.In 40% KOH electrolyte measured as milliohms-square inches.

In order to evaluate the data presented in Table I it should beappreciated that a battery separator having a resistance of 20milliohm-square inches or below is acceptable. Of course, the lower thisvalue, the better is the battery separator.

Example II which represents the imidazoline tertiary amine surfactantcoated microporous film of the prior art is included to illustrate thebest of the prior art. Of the many surfactants suggested for usage withmicroporous films in pending application Ser. No. 106,564,

cent potassium hydroxide solution and tested in accordance with theprocedure enumerated in Examples II-IV. However, the test was limited toreadings after I and 24 hours. The electrical resistivity of the threeseparate sample tests for each different coating were averaged andrecorded as milliohms-square inches. The results of these tests aresummarized in Table II. Table II includes, in addition to theinformation discussed above, the range of electrical resistance,measured after 24 hours, for the three samples similarly coated EmcolAL-42-l2, an imidozoline, represents an excel- 20 microporous films.

TABLE II VI VII VIII IX Polyoxyethylene Polymethyl Siloxane, "I: by

weight Ave. Elect. Resistance, after 1 hr.,

miIliohms-in.

Avg. Elect. Resistance,

after 24 hrs, miIliohms-in. Range, Elect.

Resistance after 24 hrs.

% Surfactant.

CL. 10.3 8.1 7.8 l3.l

O L. 7.3-[ 7,343.5 6.8-9.3 Ill-l7.

9.8 ll.6 l2 l2.4

% by weight of Overload.

lent surfactant unaffected chemically by exposure to strongelectrolytes. The results of this test clearly indicate that such asurfactant typical of the prior art, is unsatisfactory for usage after24 hours exposure of the coated battery separator in percent solution ofpotassium hydroxide. On the other hand, the two silicon glycol copolymersurfactants coated microporous films both exhibited acceptable batteryseparator resistivity within I hour of their insertion into the 40percent potassium electrolytic solution.

EXAMPLE V IX Five additional examples of uncoated polypropylenemicroporous film made in accordance with the procedure enumerated inExample I were coated by the procedure employed in Examples II-IV. Inthese examples the coating solutions, i.e., 7 percent weight solutionsin acetone, ranged from 100 percent imidazoline tertiary amine to 100percent of silicon glycol copolymer (polyoxyethylene polymethylsiloxane) surfactant of the instant invention and included three samplescomprising surfactant samples which represented the imidazoline tertiaryamine and the polyoxyethylene polymethyl siloxane silicon glycolcopolymer. Three battery separators were formed from each of the fivecoated microporous films. Each of these three samples were tested in anelectrolytic cell filled with a 40 per- The results of these testsindicate again the unsuitability of imidazoline, an otherwise excellentsurfactant for normally hydrophobic microporous film substrates, inbattery separator applications because of the slow wettability ofimidazoline coated microporous films with water. As indicated in TableII, after 1 hour and 24 hours the value of electrical resistance forimidazoline coated microporous film was so high as to be unreadable. Thesilicon glycol copolymer surfactant of the instant invention in thiscase, polyoxyethylene polymethyl siloxane, again proved suitable foremployment as a battery separator in view of its rapid wettabilityresulting in its electrical resistance being significantly below theacceptable limit of 20 milliohms-square inches.

The most surprising results were Examples Vl-VII. In these examplessilicon glycol copolymer surfactant of the instant invention was blendedwith an amidazoline tertiary amine. Surprisingly, the average electricalresistance of microporous films coated with this surfactant blend werelower even than the totally satisfactory silicon glycol copolymersurfactant coated microporous films. The best results were attainedemploying a silicon glycol copolymer imidazoline surfactant mixture.These results seem to indicate that silicon glycol copolymcrs providethe absolutely necessary instant wettability property whereasimidazoline, probably a 17 slightly better surfactant over long periods,as indicated in Table l by the 13 day electrical resistance data,provides greater hydrophilicity to the substrate.

The above preferred embodiments and examples are given to illustrate thescope and spirit of the instant invention. Other preferred embodimentsand examples within the scope and spirit, such as the use of othersurfactants blended with the silicon glycol copolymer surfactant of theinstant invention are within the contemplation of the instant invention.The instant invention, therefore, should be limited only by the appendedclaims.

What is claimed is:

l. [n a battery comprising at least one electrolytic cell, said cellincluding an anode and a cathode disposed in an electrolyte said anodeand cathode separated in said electrolyte by a battery separator theimprovement comprising the battery separator, said separator comprisinga hydrophobic, microporous polymeric film characterized by a reducedbulk density compared to the bulk density of the correspondingnon-porous precursor film, a crystallinity of above about 30 percent, anaverage pore size of about 100 to 12,000 Angstroms, a surface area ofabout 2 to about 18 200 square meters per gram, and a void volume ofbetween about 20 to 45 percent; and a surfactant coating comprisingabout 2 to 20 percent by weight of the uncoated microporous film, ofpolyoxethylene polymethyl siloxane.

2. in a battery comprising at least one electrolytic cell, said cellincluding an anode and a cathode disposed in electrolyte, said anode andcathode separated in said electrolyte by a battery separator theimprovement comprising the battery separator, said separator comprisinga hydrophobic, microporus, polymeric film characterized by a reducedbulk density compared to the bulk density of the correspondingnon-porous precursor film, a crystallinity of above about 30 percent, anaverage pore size of about and 12,000 Angstroms, a surface area of about2 to about 200 square meters per gram, and a void volume of about 20 to45 percent; and a surfactant coating comprising about 2 to 20 percent,by weight of the uncoated microporous film, said coating comprising amixture of a polyoxethylene polymethyl siloxane and an imidazolinetertiary amine.

1. IN A BATTERY COMPRISING AT LEAST ONE ELECTROLYTIC CELL, SAID CELLINCLUDING AN ANODE AND A CATHODE DISPOSED INAN ELECTROLYTE SAID ANODEAND CATHODE SEPARATED IN SAID ELECTROLYTE BY A BATTERY SEPARATOR THEIMPROVEMENT COMPRISING THE BATTERY SEPARATOR, SAID SEPARATOR COMPRISINGA HYDROPHOBIC, MICROPOROUS POLYMERIC FILM CHARACTERIZED BY A REDUCEDBULK DENSITY COMPARED TO THE BULK DENSITY OF THE CORRESPONDINGNON-POROUS PRECURSOR FILM, A CRYSTALLINITY OF ABOVE ABOUT 30 PERCENT, ANAVERAGE PORE SIZE OF ABOUT 100 TO 12,000 ANGSTROMS, A SURFACE AREA OFABOUT 2 TO ABOUT 200 SQUARE METERS PER GRAM, AND A VOID VOLUME OFBETWEEN ABOUT 20 TO 45 PERCENT; AND A SURFACTANT COATING COMPRISINGABOUT 2 TO 20 PERCENT BY WEIGHT OF THE UNCOATED MICROPOROUS FILM, OFPOLYOXETHYLENE POLYMETHYL SIOXANE.
 2. In a battery comprising at leastone electrolytic cell, said cell including an anode and a cathodedisposed in electrolyte, said anode and cathode separated in saidelectrolyte by a battery separator the improvement comprising thebattery separator, said separator comprising a hydrophobic, microporus,polymeric film characterized by a reduced bulk density compared to thebulk density of the corresponding non-porous precursor film, acrystallinity of above about 30 percent, an average pore size of about100 and 12,000 Angstroms, a surface area of about 2 to about 200 squaremeters per gram, and a void volume of about 20 to 45 percent; and asurfactant coating comprising about 2 to 20 percent, by weight of theuncoated microporous film, said coating comprising a mixture of apolyoxethylene polymethyl siloxane and an imidazoline tertiary amine.